1. Field of the Invention
The present invention pertains to seismic data processing and more particularly to processing data representing acoustic seismic (visco-elastic) waves reflected from both a substantially horizontal interface and a steeply dipping interface.
2. Related Prior Art
In acquiring seismic data, a seismic source is used to generate visco-elastic seismic acoustic waves that are directed into the earth's surface. At changes in the impedance of subsurface material, commonly called interfaces, the waves are partially reflected back in the direction of the seismic source at an angle governed by the law of reflection. The reflected portions of the waves are detected by seismic receivers. These receivers produce an electrical signal representing the reflected wave which may then be digitized and recorded. Further processing uses the recorded electrical signals to calculate the travel time of the wave, approximate the average velocity of the wave and determine the depth of the subsurface interface. In general, it is assumed that the recorded electrical signals represent (seismic) acoustic waves which have been reflected once.
In general, prior art methods are incapable of determining the location of steeply dipping interfaces through surface generation of acoustic or seismic pulses. Prior art methods for approximating steeply dipping interfaces require subsurface seismic pulse generation, using single bounce characteristics in data processing.
For example, U.S. Pat. No. 4,509,149 titled "Directional Long Array for Logging Vertical Boundaries" (Ruehle) deals with an apparatus used to plot typical interfaces. A logging sonde has directional control of multiple sources and receivers in linear arrays. Each of the sources produces acoustic pulses which are delayed by a time delay between the sources to direct acoustic energy at an angle such that the resulting acoustic wave strikes a vertical formation or interface. Reflections of the acoustic pulses are detected with a linear array of receivers. The acoustic pulse directed by each receiver is delayed by an amount such that the total differential moveout for a reflected directional wave is zero.
This patent is similar to the present invention in so far as it is used to determine the location of nearly vertical interfaces. However, Ruehle uses data obtained from a downhole logging tool having subsurface acoustic pulse sources and receivers in defining the shape of a subsurface vertical formation. Further, Ruehle uses single bounce characteristics in data processing to identify the location of the vertical or nearly vertical interface.
U.S. Pat. No. 4,346,462 "Seismic Prospecting Method Providing for the Determination of the Flank Dip of Subterranean Layers and Device for Carrying Out the Same" (du Boullay) uses a surface pulse generator to transmit acoustic waves with different directivity diagrams toward a subsurface interface. This patent describes a method for determining the flank dip of subterranean layers with respect to the plane of a seismic profile. Acoustic waves with different directivity diagrams are sequentially transmitted towards subsurface interfaces. These corresponding reflected waves are recorded. Transmitted acoustic intensities are combined according to predetermined criteria. Values which are representative of the intensity of the reflected waves are combined separately in each of the directivity diagrams. The results of the separate combinations are then compared.
Du Boullav provides a method for determining the flank dip of subterranean layers with respect to the plane of a seismic profile by using a single bounce characteristic in determining the position of the flank dip. While the method described can identify dipping formations, it requires specially generated acoustic pulses and uses pulses that have been reflected only once.
U.S. Pat. No. 3,696,331 titled "Automated Process for Determining Subsurface Velocity from Seismograms" (Guinzy et al.) deals with a method for determining the velocity of acoustic pulses in a subsurface formation from a suite of seismograms. The signal power of windows from the seismograms is detected for different assumed values of velocity, vertical travel time and dip. A function indicative of signal presence is plotted as a function of velocity for different vertical travel times to provide an indication of the acoustic velocity characteristic. The plot of signal power for different dip searches provides seismograms from an area including dipping formations.
The Guinzy et al. patent provides a method for determining the velocity of subsurface acoustic pulses for a suite of seismograms. In determining the velocity, different values of dip are assumed for a dip search. A plot of signal power for different dip searches is used to provide seismograms for an area which includes dipping formations. As with the previous two patents, the Guinzy et al. patent also relies on single bounce characteristics.
U.S. Pat. No. 3,668,620 titled "Method and Apparatus for Determination of Seismic Signal Event Coherence" (Mathieu et al.) relates to a method and apparatus for processing multi-trace seismic signals to determine the most coherent dip attitude for any selected point in time. Similar event signals from a plurality of seismic traces at selected time delays per trace in predetermined time increments along the multi-trace seismic signals are accentuated. Each of the plural traces of selected time delays per trace are combined to derive a plurality of signals, each indicative of a selected step-out or dip angle. The plurality of signals are combined to produce an output signal indicative of the most coherent dip attitude.
While this method is particularly appropriate for subsurface formations having slight to moderate dip, this method along with all of the foregoing methods using surface sources, is incapable of dealing with steeply dipping formations. None of the methods make use of seismic energy which reflects from both a nearly horizontal interface and a nearly vertical interface.